Automatic frequency control circuit



March 21, 1939. w. F. EWALD ET AL AUTOMATIC FREQUENCY CONTROL CIRCUIT Filed Dec. 15, 1937 FQSEEL AJ-I AMPL.

1 INVENTOR WOLFGANG F- I F AMPL I. RAM/ L.

SOURCE S FWALD KARL W/LHELM ATT QRNEY Patented Mar. 21, 1939 UNITED STATES PATENT, OFFICE} AUTOMATIG FREQUENCY CONTROL CIRCUIT Germany Application December 15, 1937,Serial No. 179,883 In Germany November 14, 1936 3 Claims. (01. 250-20) Arrangements are known in the prior art which have the purpose to insure automatic correction of tuning of receiving apparatus for amplitudemodulated oscillations, after rough or coarse adjustment of the tuning means to the frequency channel of the station to be received has been effected manually. For this object recourse is usually had to differential circuit organizations comprising two intermediate frequency (IF) circuits which are detuned a certain amount in ref erence to each other, or the potentials of which present a phase difference, or shift, in reference to each other and in which a differential potential arises as long as the carrier IF differs from the rated, or proper, value. The rectified differential potential in these circuit organizations serves for the direct, or indirect, shift of the oscillator frequency by means of a variation of the tube capacities, of inductances or by mechanical adjustment of a variable condenser. Now, all of these schemes have the drawback that for the production of the desired effect several tubes and several tuned circuits are additionally required. This fact confines this arrangement to receiver sets in which cost of production plays no decisive part.

The present invention is predicated upon a circuit organization disclosed in an earlier invention by Ewald et al. in application Serial No. 179,882, filed December 15, 1937, adapted to insuring automatic sharp tuning in. which the regulator potentials being in differential relationship to each other are obtained from a band-pass filter, the secondary coil of which has its midpoint connected with the high potential end of the primary coil. According to the present invention the two rectifiers serving to produce the differential regulator potentials are used at the same time for demodulation in that, by the use of a transformer, the phase of the AF potential obtained from one of the rectifiers is shifted.

In the drawing:

Fig. 1 shows, by way of example, an arrangement comprising a single power tube; while Fig. 2 shows an arrangement comprising a push-pull power stage.

Referring to the drawing, l denotes the usual IF part of a superheterodyne receiver set. band-pass filter 3 feeds the two diodes 2 and I3; across the load resistances of the latter indicated at 4 and H, the opposite or differential regulator potentials are obtained. The choke coil it closes the D. C. path of the diodes. This is a well known discriminator network.

In order, additionally, to obtain the audio ener- The gy from the diode arrangement for the' regulator potential, the windings 28 and 29 of an AFtransformer are connected in series with the said load resistances 4 and IT. The lower'end of the secondary winding 29 is brought to a point 3| where the regulator potential is obtained. The same is amplified in the power tube, and serves to shift the corrector condenser 2| of the heterodyne by the agency of an electrodynamic means 2|.

The two halves of the primary winding must be Wound in opposition in order that potentials induced in the secondary may not neutralize each other. The parallel condensers connected to 4 and I! must be enlarged so that they will have a short-circuiting action so far as the AF is concerned. The alternating current flowing through the diode will not increase to a point exceeding the D. C., since the large inductances of the transformer are connected in series with the load condensers. It is expedient to by-pass the series system by the condensers 32 and 33 for RF.

In the scheme shown in Fig. 2, AF transformer windings must be connected in series with the load resistances of the two diodes. The two ends of the secondary windings 3030' are brought to the grids of the push-pull stage The middle thereof is broken open, and the resultant two halves must be connected with the respective ends of the load resistance away from the cathode. Sofar as the load and by-pass condensers (which bear the same reference numerals as in Fig. 1) as well as the sense of winding of the transformer are concerned, there applies what has been point ed out above. The coils 2| of the electrodynamic drive mechanism for the corrector condenser in the present exemplified embodiment are disposed in the plate circuit of the power tubes near the filaments.

What is claimed is:

1. In a superheterodyne receiver of the type comprising an intermediate frequency network and an audio amplifier network; a detection network comp-rising a pair of rectifiers having a common frequency intermediate input circuit coupled to said intermediate network, a load resistor connected to one rectifier developing a direct current voltage thereacross, a second load resistor connected to the second rectifier and developing a direct current voltage, said two resistors being arranged in polarity opposing relation, means for impressing the differential of said voltages on said audio network means responsive to the current flow of the latter for maintaining the intermediate energy at an assigned frequency value, and audio coupling means between each rectifier and said audio network for impressing on the latter the audio voltage output of each rectifier whereby said audio network simultaneously amplifies the audio voltages and said differential voltage.

2. In a superheterodyne receiver of the type comprising an intermediate frequency network and an audio amplifier network; a detection network comprising a pair of rectifiers having a common frequency intermediate input circuit coupled to said intermediate network, a load resistor connected to one rectifier developing a direct current voltage thereacross, a second load resistor connected to the second rectifier and developing a direct current voltage, said two resistors being arranged in polarity opposing relation, means for impressing the differential of said voltages on said audio network, means responsive to the current flow of the latter for maintaining the intermediate energy at an assigned frequency value, and audio coupling means between each rectifier and said audio network for impressing on the latter the audio voltage output of each rectifier whereby said audio network simultaneously amplifies the audio voltages and said differential voltage, said'audio coupling means comprising an audio transformer having a pair of primary windings and a secondary winding, said primary windings being poled in additive phase.

3. In a superheterodyne receiver of the type comprising an intermediate frequency network and an audio amplifier network; a detection network comprising a pair of rectifiers having a common frequency intermediate input circuit coupled to said intermediate network, a load resistor connected to one rectifier developing a direct current voltage thereacross, a second load resistor connected to the second rectifier and developing a direct current voltage, said two resistors being arranged in polarity opposing relation, means for impressing the differential of said voltages on said audio network, means responsive to the current flow of the latter for maintaining the intermediate energy at an assigned frequency value, and audio coupling means between each rectifier and said audio network for impressing on the latter the audio voltage output of each rectifier whereby said audio network simultaneously amplifies the audio voltages and said differential voltage, said audio network comprising a pair of tubes arranged in push-pull relation.

WOLFGANG F. EWALD. KARL WILHELM. 

